(1)
The isospin is a good symmetry for the form
factor \(F_{0,1}^{B_{c}{\rightarrow }B_{u}}\)  \(=\) 
\(F_{0,1}^{B_{c}{\rightarrow }B_{d}}\) , including the fitted pole
mass \(m\)  and curvature parameter \({\delta }\) .
Considering the uncertainties, the values of form
factors \(F_{0,1}^{B_{c}{\rightarrow }B_{q}}\)  at the pole
\(q^{2}\)  \(=\)  0 are consistent with the recent results
estimated with the relativistic independent quark model,
where \(F_{0,1}^{B_{c}{\rightarrow }B_{u,d}}(0)\)  \(=\)  \(1.01\) 
and \(F_{0,1}^{B_{c}{\rightarrow }B_{s}}(0)\)  \(=\)  \(1.03\) 
[1]}. As it is well known,
the spectator is the heavy \(b\)  quark in the
\(B_{c}\)  \({\rightarrow }\)  \(B\)  transition.
The velocity of the \(B\)  meson is very low
in the rest frame of the \(B_{c}\)  meson.
The wave functions of \(B_{c}\)  and \(B\)  mesons
overlap severely, which result in the
large \(B_{c}\)  \({\rightarrow }\)  \(B\)  transition form factors.
